Tony Juniper, the Green party's candidate for Cambridge, is campaigning on a bicycle with a wheelbarrow-shaped front carriage, sturdy enough to carry leaflets weighing up to a quarter of a tonne. The leaflets are printed on 100% recycled paper, using, where possible, vegetable dyes. The electricity in his headquarters comes from a company that supplies renewable energy, and the wood for his campaign billboards comes from sustainable forests.

The former director of Friends of the Earth hopes to inspire the whole of Cambridge to adopt similar values and has a vision of transforming the city into a model of environmental sustainability, with residents living in solar-powered homes, bicycling to local shops and eating more locally produced food.

After a slow start, Juniper believes the campaign in central Cambridge has become a four-way race. Two online polls conducted by the Cambridge News have this week put the Greens ahead. One published on Monday said that if the city followed the trend of the paper's respondents (who come from a much wider geographical area, across Cambridgeshire) the Greens would be first past the post with 24.2% of the vote, followed by the Liberal Democrats' candidate, Julian Huppert, with 23%.

Online polls are unreliable, however, and politicalbetting.com today had Juniper at around 50/1 to win, favouring the Liberal Democrats instead. Both Huppert and the Labour candidate, Daniel Zeichner, dismiss the newspaper's poll, but they concede that the Green party's support has strengthened locally.

Cambridge has no core loyalty to any party, leaving all candidates fighting hard for the undecided voters.

The constituency was Conservative until 1992, when it was taken by Labour, who held it until 2005. Labour support disintegrated amid disaffection over the Iraq war and the seat was won in the last election by a Lib Dem, David Howarth, who has now stood down. With the race so open, candidates from all parties are participating in an intense schedule of well-attended public debates.

The Green campaign has been aided by a benefit gig performed by Radiohead's Thom Yorke and an appearance by Ken Livingstone, who did not formally back Juniper but said it would be good to see Green MPs in parliament.

Juniper talks as much about the economy and social justice as issues related more directly to the environment, working to convince voters that this is a party with a strong position on all issues. But his central argument hangs on the need to combine mending the economy and addressing the environment.

"The other parties say we need to do the economic growth now and the environment later. That is how the conversation has gone for 30 years. I fear that if we don't get some Greens in there soon, that's how it will continue," he said.

"We will have 9 billion people living on Earth by 2050 at a time when we have to cut greenhouse emissions by 80%. You can't do it with everybody living like Europeans. And you won't be able to do it politically if you have a few people living like Europeans and everybody else living in abject poverty. The scale of the transformation needed is unprecedented."

Cutting carbon emissions at a local level can be done by creating a greater sense of community, and encouraging local, sustainable farming and business.

"We will have local people trading with each other, more people having a direct link with the people who provide their services," he said.

Nationally, the Green party is focusing on winning three seats – and Cambridge is not one of them.

The Lib Dems' local website has a graph on its front page saying "Cambridge is a two-horse race", and pointing out that the Lib Dems took 44% of the vote in the last election, while Labour took 34%, and the Greens 2.9%.

"We have been canvassing data from thousands of voters around the constituency showing that Cambridge continues to be a Labour/Lib-Dem marginal, as it has been for a decade," Huppert said.

The Greens' share of the vote in the last election was low, Juniper argues, because they did not mount an organised campaign. This time they have an energetic team in the constituency, with more than 120 campaigning volunteers. "It is an audacious punt, it is ambitious, but I think we can win," he said.

Zeichner thinks it unlikely. "I wouldn't attach too much significance to an online poll, but there is no doubt that the Greens are going to do well. I would describe Cambridge as a three-and-a-half-way marginal," he said.